Method for the production of green liquor

ABSTRACT

The method is for production of green liquor in association with a causticizing process during recovery of chemicals in manufacture of sulphate pulp. A smelt ( 14 ) of chemicals principally consisting of Na2S and Na 2 CO 3  from a soda boiler ( 1 ) is provided. Weak liquor ( 15 ) that contains dissolved NaOH and CaO is provided. A dissolving tank ( 2 ) is provided in which the smelt ( 14 ) from the soda boiler ( 1 ) is dissolved in the weak liquor ( 15 ) it order to form green liquor ( 16 ). A solution of NaOH is added to the weak liquor ( 15 ), at a position before the weak liquor ( 15 ) is added to the dissolving tank, in order to increase the concentration of NaOH in the weak liquor ( 15 ) such that a fraction of CaO that is presently dissolved in the weak liquor ( 15 ) precipitates.

PRIOR APPLICATION

This application is a U.S. national phase application based onInternational Application No. PCT/SE2003/001806, filed 21 Nov. 2003,claiming priority from Swedish Patent Application No. 0203478-3, filed25 Nov. 2002.

TECHNICAL AREA

The present invention concerns a method for the production of greenliquor in association with a causticizing process during the recovery ofchemicals in the manufacture of sulphate pulp. The method comprises theprovision in a first step of a smelt of chemicals that principallyconsists of sodium sulphide (Na₂S) and sodium carbonate (Na₂CO₃) from asoda boiler; and the provision in a second step of a weak liquor thatcontains, among other compounds, dissolved sodium hydroxide (NaOH) anddissolved calcium oxide (CaO). Furthermore, the method comprises theprovision in a third step of a dissolving tank in which the smelt fromthe soda boiler is dissolved in weak liquor in order to produce greenliquor.

THE PRIOR ART

It has been known for a long time that, in factories for the manufactureof sulphate paper, pulp deposits, known as encrustations, in the processequipment lead to a reduction in the production capacity of the plant.The deposits can, for example, be constituted in boiler equipment and inevaporation equipment by calcium carbonate and sodium carbonate, orcalcium sulphate and sodium sulphate, or by various compounds of siliconor aluminium. These substances arise partly from the process fluid, i.e.the white liquor, while precipitated substances from wood alsocontribute to the problems.

Deposits on process equipment in the chemical recovery system of thefactory also cause a reduction in the degree of exploitation of theplant. For example, the deposits that are formed in the dissolver, wheresmelt from the soda boiler is dissolved in an aqueous solution known asweak liquor in order to form green liquor, give rise to a number ofproblems. It may be necessary, for example, to reduce the capacity ofthe soda boiler as a result of a reduced circulatory efficiency in thedissolver. Deposits in pipes and in pumps for the onwards transport ofthe green liquor from this tank can also contribute to reducing thedegree of exploitation of the plant. A further negative effect of thisis that it may not be possible in the worst case to maintain the densityof the green liquor, which is an indirect measure of its chemicalcontent, at a stable value, and the density may vary. This causesproblems in the subsequent process steps which comprise, among othersteps, slaking and causticizing, something that results in unevenquality of the white liquor.

U.S. Pat. No. 4,302,281 (1981) reveals a process for reducing theproblems with precipitation and deposition in pulp-producing equipmentby reducing the amount of inactive compounds, known as ballastcompounds, in the white liquor. However, the document does not discussthe problem of deposits from weak liquor. Furthermore, this methodresults in the sludge that is formed also containing NaSH, which is alsodissolved in the weak liquor, after which this NaSH in the weak liquoris oxidised, leading to an undesired ballast in the factory.

A process is revealed in U.S. Pat. No. 4,536,253 (1985) for themeasurement of the properties of the white liquor in order thereby tomake possible a more efficient use of chemicals while at the same timereducing problems caused by uneven quality of the while liquor. Theproblems that are described are wholly related to the quality of thewhite liquor and do not name any problems related to the addition of theweak liquor to the dissolver.

A process is described in U.S. Pat. No. 5,213,663 (1993) for theregulation of the concentration of sodium carbonate in the green liquorin the dissolving tank. This process constitutes an example of how theability of the weak liquor to dissolve deposits that have already formedcan be used, but the process does not tackle the root of the problem,i.e. preventing or minimising the formation of deposits.

Another process that deals with the problem of deposits from greenliquor in the dissolving tank and in pipes connected to it and in otherequipment for transport of green liquor is described in U.S. Pat. No.5,820,729 (1998). This process is a further example of how the abilityof weak liquor to dissolve deposits is used, but, neither does thisdocument present a method for the prevention or minimisation of theformation of these.

Furthermore, it is previously known through, among other sources, anarticle of W. J. Frederick Jr. and Rajeev Krishnan “Pirssonite depositsin green liquor processing”, published in the TAPPI Journal in February1990, that pirssonite deposits in the dissolver can be reduced byallowing the sludge to be transported from the sludge wash together withthe weak liquor. The method, however, generates an undesirable loss ofsludge since the particles that are transported from the sludge wash areled to the dissolver for subsequent separation and dumping as sludgefrom the green liquor clarification/filtration, rather than being led tothe sludge oven in order to be further processed to quicklime CaO.

BRIEF DESCRIPTION OF THE INVENTION

It is one aim of the present invention to offer a method thatessentially eliminates or at least minimises a number of the negativeeffects that are caused by deposits in the dissolving tank. This isachieved through the addition of a solution of sodium hydroxide to theweak liquor at a position before the weak liquor is added to thedissolving tank, in order in this way to increase the concentration ofsodium hydroxide in the weak liquor such that a fraction of the calciumoxide that is dissolved in the weak liquor precipitates.

The invention allows the degree of exploitation of the chemical recoveryto be maintained at a high level both with respect to the soda boilerand the production of green liquor in the dissolver. Availability alsoincreases in that the maintenance requirement is reduced as a result ofthe lower degree of deposits in the dissolver and on the pipes, pumpsand other equipment that is normally exposed and requires cleaning,either mechanical, chemical or a mixture of both, at regular intervals.

The invention can be explained in that there are two types of depositsin the dissolver and in pumps and pipes connected to it. One type isconstituted by pirssonite, Na₂Ca(CO₃)₂-2H₂O. This type of deposit iswell-known and arises principally as a result of too high a density ofthe green liquor. It is for this reason important to monitor carefullythe density on exit from the dissolver. The second type is constitutedby calcium carbonate, CaCO₃, which deposits have until now not receivedattention to the same degree as pirssonite. These deposits can be causedby the fact that the content of dissolved sodium hydroxide in the weakliquor nowadays can be very low, typically 5-10 g/l. The content ofcalcium oxide dissolved in the weak liquor can, at these low levels ofsodium hydroxide, be significant. One reason for low levels of sodiumhydroxide is the presence in modern plants of efficient filters for theseparation of white liquor and sludge. This results in most of thealkali being removed as white liquor, with very little accompanying thesludge and arriving at the weak liquor. In older causticizing plants, onthe other hand, the level of sodium hydroxide in the weak liquor wassignificantly higher, typically >20 g/l. The solubility of calcium oxidein weak liquor is, however, very low at such a concentration of sodiumhydroxide, indeed, it is essentially zero. Thus, addition of weak liquordid not cause the formation of such deposits in the dissolving tank, atleast, not to such a degree that negative results from the depositsarose.

The advantage is achieved through the invention that the dissolvedcalcium oxide is precipitated before the weak liquor reaches the greenliquor in the dissolver. For this reason, the calcium oxide cannot giverise to deposits/“encrustations” in the dissolver or in subsequent pumpsand pipes. Furthermore, a synergistic effect is at the same timeachieved in that the precipitated particles form “growing surfaces”,which ensure that any pirssonite that does form does not precipitateonto the surfaces of the equipment, but rather on the “growingsurfaces”.

A further advantage is achieved with the invention in that theconcentration of sodium hydroxide in the weak liquor from the washedsludge is maintained at a level below 20 g/l, preferably under 15 g/land even more preferably under 10 g/l before any addition of sodiumhydroxide is carried out, and in that the addition of sodium hydroxideto the weak liquor is carried out at such a level that the concentrationof sodium hydroxide in the weak liquor increases at least 20%,preferably at least 40% and even more preferably at least 60%. Thiscontributes to a more efficient use of the sodium hydroxide than thatachieved by older processes in which the concentration of sodiumhydroxide in the weak liquor is naturally high through the highercontent of sodium hydroxide in the sludge.

BRIEF DESCRIPTION OF FIGURES

The invention will be described in more detail with reference to theattached drawings where:

FIG. 1 shows a flow diagram of a section of a causticizing plant inwhich the invention is used, and

FIG. 2 shows a diagram of the solubility of calcium oxide (CaO) as afunction of the concentration of sodium hydroxide (NaOH).

DETAILED DESCRIPTION OF FIGURES

FIG. 1 shows a flow diagram of a plant for chemical recovery in asulphate pulp factory. The figure also shows schematically how theinvention is used in one preferred embodiment. The chemical recoverycomprises the following equipment: soda boiler 1, dissolver 2, greenliquor clarifier 3, sludge filter 4, lime slaker 5, causticizing vessel6 a, 6 b, white liquor filter 7, sludge filter (atmospheric) 9, andsludge oven 10. In addition to this equipment there are pipes, pumps,tanks, measurement equipment, regulator equipment and other equipmentthat is well-known to one skilled in the arts.

The soda boiler 1 is a steam boiler that has been adapted to burn blackliquor. It is also a chemical reactor and it constitutes the first stepin the conversion of chemicals recovered in the pulp washing to newcooking chemicals. The liquor after evaporation (thick liquor) issprayed into the oven of the soda boiler through special nozzles. Airfor combustion is blown in, and the liquor is dried by the hot exhaustgases, forming a bed in the bottom of the oven. The organic matter isvaporised in this bed, and is exhaustively combusted at a higher levelin the oven. Carbon dioxide that is produced forms sodium carbonate(Na₂CO₃)—soda—with a fraction of the sodium present in the dry matter,and it is this that has led to the soda boiler being so named. Sodiumsulphate (Na₂SO₄) formed, together with any sodium sulphate that may beadded as make-up chemical, is converted by reduction with the aid ofcarbon in the carbonised dry matter to sodium sulphide (Na₂S). Sodiumsulphide is an active cooking chemical.

Sodium carbonate (Na₂CO₃), sodium sulphide (Na₂S) and some ballastchemicals run out from the bottom of the oven in the form of smelt 14,down into the dissolver 2. The smelt 14 is there dissolved in what isknown as weak liquor 15 from the white liquor preparation and is thencalled green liquor 16. The green liquor 16 from the soda boiler 1passes onwards to the white liquor preparation, which is the finalprocess step in the recovery system. The sodium sulphide in the smelt 14is present in the green liquor 16 as sodium hydroxide and sodiumhydrogen sulphide. These substances pass through the white liquorpreparation without, in principle, being modified. The sodium carbonatemust be converted to sodium hydroxide. Most of the sodium hydroxide inthe white liquor is formed in this conversion process.

Green liquor 16 from the dissolver 2 is first cleaned of contaminants inthe form of sludge. This takes place in the chart in FIG. 1 in the greenliquor clarifier 3, which separates out the sludge throughsedimentation. Pressure filters have also recently begun to be used. Anewly developed piece of apparatus is a tube filter known as a “cassettefilter”. Green liquor 16 that remains in the separated sludge is washedout through a sludge filter 4. The filtrate (weak liquor) is pumped tothe dissolver 2 of the soda boiler 1.

The cleaned green liquor 16 passes to the lime slaker 5 in which it ismixed with quicklime. The water in the green liquor 16 reacts with thelime, which becomes calcium hydroxide. Sand and unreacted fragments oflime are scraped out and taken away to a landfill. The “causticizationreaction” also commences in the slaker. The mixture of liquor and limepasses from the lime slaker 5 to several causticization vessels 6 a, 6b, connected in series. The reaction between sodium carbonate andcalcium hydroxide continues there. The products are sodium hydroxide andcalcium carbonate (CaCO₃). Sodium hydroxide is also known as causticsoda, and this is the reason that the reaction is known by the name“causticizing”. Only approximately 80% of the sodium carbonate can beconverted to sodium hydroxide since the reaction is an equilibriumreaction.

After the causticizing, sodium hydroxide has been reformed and theliquor is once again white liquor. The other reaction product, calciumcarbonate, which is also known as sludge, must be removed before theliquor can be used. The white liquor is ready to be reused in thedigester once the sludge has been removed.

The separation of the sludge, clarification of the white liquor, takesplace in modern filter equipment using pressure disc filters. The sludgethat is separated is often washed partially in the disc filter using,for example, hot water. This takes place using a spray from washingnozzles inside the filter. Separated sludge can then be diluted with,for example, condensate from evaporation or with hot water. The washedand diluted sludge is stored in a storage tank. It is pumped from thereto a sludge filter 9 of vacuum drum type. Final washing takes placethere, and drying to a value of approximately 75% dry matter. The sludgeis then transferred onwards to a sludge oven 10 in which it is reburnedto lime. The sludge oven 10 is a rotation oven that comprises a long,slightly inclined tube. The sludge is fed into the upper end of theoven. Fuel, which may be oil, gas, pine oil, methanol from cleaning ofthe condensate or biofuel, is added at the lower end. Sludge is feddownwards towards the combustion zone, where the temperature isapproximately 1 200° C., by the rotation. Drying, calcination (i.e.conversion of the calcium carbonate to calcium oxide), and sinteringtake place in the oven. In this way, the closed cycle of lime recoveryis complete.

In order to apply the invention, it is principally the dissolver 2 and atank 11 for weak liquor 15 that are the equipment that is taken intoservice in order to apply the invention, something that has also beenshown in FIG. 1 within the dashed region. It is to be understood that itcan be an advantage if this tank 11 is constituted by a storage tankthat already exists for the storage of weak liquor 15, but a new tankmay, of course, be installed for this function. In the method that isrelevant for the invention, washing fluid from the sludge wash 8 andfrom the green liquor sludge wash 4 form a weak liquor that is led tothe dissolver 2. Various forms of process water, such as condensate fromthe evaporation plant, for example, or hot water, may also be added tothe weak liquor. A sludge with a low concentration of NaOH is obtainedin causticizing plants in which modern high-yield filters have beeninstalled as white liquor clarifiers 7 for separating the sludge fromthe white liquor. The solubility of calcium oxide depends heavily on theconcentration of sodium hydroxide in the weak liquor 15, and a low levelof NaOH makes a high level of CaO possible. This relationship is shownin the diagram in FIG. 2.

A pipe 12 is connected to the tank 11 for the addition of sodiumhydroxide to the tank. By adding sodium hydroxide to the weak liquor 15in such an amount that the concentration of sodium hydroxide in the weakliquor 15 increases by at least 20%, preferably at least 40%, and mostpreferably at least 60%, can fractions of the calcium oxide in the weakliquor 15 be precipitated out. The precipitate is constituted by calciumhydroxide (Ca(OH)₂), and Ca(OH)₂ can be converted to calcium carbonateCaCO₃ in solid form, depending on the composition of the weak liquor 15.This precipitation takes place preferably in the tank 11 and theaddition of sodium hydroxide is preferably arranged in such a mannerthat the precipitation takes place as small particles in the solution.It is an advantage if these small particles are allowed to accompany theweak liquor 15 to the dissolver 2 in order there to form growingsurfaces for the precipitation of the remaining quantity of calciumoxide in the weak liquor 15. A certain amount of calcium oxide can beseparated out by sedimentation, depending on how much calcium oxide hasbeen precipitated in the tank 11. The separated precipitate can be ledonwards to the sludge silo 13, in order in this way to pass onwards tothe sludge oven 10 for conversion to quicklime. If it is decided toallow the particles to pass to the dissolver, it is an advantage if theweak liquor tank 11 is provided with a stirrer.

As can be seen in the diagram in FIG. 2, addition of sodium hydroxide(NaOH) to the weak liquor 15 will have effects on the precipitation ofcalcium oxide (CaO) of different magnitude, depending on theconcentration of NaOH in the weak liquor 15 prior to the addition ofNaOH. Greatest effect is achieved for weak liquors having a lowconcentration of NaOH, where the solubility of CaO is high. Theinvention is thus particularly suitable where the concentration of NaOHin the weak liquor 15 in the weak liquor tank 11 lies below 10 g/lbefore any addition of NaOH is carried out. However, weak liquors withNaOH concentrations up to 20-25 g/l can also be given an addition offurther NaOH, although the effect of this addition will not in this casebe as great.

For example, in weak liquors with an NaOH concentration of 5 g/l, anincrease of the level to 8 g/l will cause 30 g CaO/m³ to beprecipitated, given that the weak liquor was saturated with CaO beforethe addition. This precipitation corresponds to approximately 75% of thetotal amount of dissolved CaO in the weak liquor. If the weak liquorrather has an NaOH concentration of 10 g/l, an increase to 13 g/l willlead to the precipitation of approximately 3 g CaO/m³ weak liquor. Aweak liquor with an NaOH concentration of 15 g/l will give aprecipitation of approximately 1 g CaO/m³ weak liquor when theconcentration is increased to 18 g/l.

In one example in which the invention is used in one preferredembodiment in a sulphate pulp factory, the production of green liquoramounts to 5,000 m³ per day. Weak liquor is obtained from the sludgewash with a temperature of 70° C. and with an NaOH concentration of 5kg/m³. This weak liquor can theoretically dissolve 40 g CaO/m³. If theNaOH concentration is increased to 8 kg/m³ by the active addition ofNaOH, in, for example, the weak liquor tank 11, without the temperatureof the weak liquor being changed, then 75% of the dissolved CaO will beprecipitated. The remaining 25%, which corresponds to 10 g CaO /m³,remains dissolved in the weak liquor and will be precipitated when theweak liquor is added to the dissolver 2. In the absence of preparationof the weak liquor according to the invention, 200 kg of CaO per day isprecipitated in the dissolver, while 150 kg will be precipitated in theweak liquor tank 11 in the form of small particles of calcium hydroxide(Ca(OH)₂) and calcium carbonate (CaCO₃) when using the inventionaccording to the example given above. The remaining 50 kg will beprecipitated in the dissolver and preferably onto the precipitatedparticles that have been allowed to accompany the weak liquor from theweak liquor tank 11.

The solubility of CaO in the weak liquor 15 depends on its temperature,in addition to the concentration of NaOH, in such a manner that thesolubility decreases with increased temperature. The invention alsocomprises the use of this relationship in order to precipitate CaO. Itis thus possible to precipitate CaO by increasing the temperature of theweak liquor 15. The method is, however, limited in that the temperatureof the weak liquor 15 when it is led to the dissolver 2 must lie at sucha level that boiling is avoided in the dissolver 2. It is thus anadvantage when using the invention that the temperature of the weakliquor 15 is taken into consideration.

In order to use the method for precipitating CaO that is describedabove, a separate step can be arranged in one variant of the inventionin which the temperature is raised during a first phase in order toprecipitate CaO, after which the weak liquor 15 is in a later phasecooled to a temperature suitable for its addition to the dissolver 2. Itis possible that precipitated Ca(OH)₂ must be separated from the weakliquor before the cooling phase, in order to prevent it redisolving.

The invention is not limited to that which has been described above, butcan be varied within the framework of the attached claims. For example,it will be apparent that sodium hydroxide can be added to the weakliquor 15 at another position than at the tank 11. It is appropriatethat sodium hydroxide is added to the weak liquor after the green liquorsludge has been separated at the sludge filter, or the sludge has beenseparated from the sludge filter. It may, for example, be advantageousto establish a separate tank for this purpose when it is desired toremove a part of the precipitate through sedimentation. It is,naturally, possible to separate the precipitate from the weak liquor 15by other separation methods familiar to one skilled in the arts, byfiltering, for example, but it is to be understood that how this isaccomplished is not a significant characteristic of the invention.

It will be apparent with reference to the diagram in FIG. 2 that theinvention can also be used for weak liquors with concentrations ofsodium hydroxide that approach 25 g/l.

The addition to the weak liquor 15 is preferably constituted by asolution of sodium hydroxide, but one skilled in the arts will realisethat the invention also allows the addition to be constituted by otheralkali solutions such as, for example, white liquor, green liquor, or amixture of these solutions.

It is also conceivable within the innovative concept of the invention toadd the sodium hydroxide to one of the filtrates that will constitutepart of the weak liquor. It is preferable that this filtrate has no, ora very low, content of CaO, in order to prevent precipitation ofCa(OH)₂.

It is advantageous if the addition of alkali to the weak liquor 15 iscarried out in a tank, but the innovative concept also comprises anaddition of alkali in the line that transports weak liquor 15 to thedissolver 2. It is also conceivable to arrange the addition in thedissolver 2 above the fluid surface such that the precipitation takesplace at this location. In this case, however, if these positions forthe addition are selected, the advantage is not achieved, or thepossibility is made more difficult, to preserve the precipitate forrecovery of chemicals.

While the present invention has been described in accordance withpreferred compositions and embodiments, it is to be understood thatcertain substitutions and alterations may be made thereto withoutdeparting from the spirit and scope of the following claims.

1. A method for the production of green liquor in association with acausticizing process during recovery of chemicals during the manufactureof sulphate paper pulp, comprising: providing a smelt of chemicalsconsisting essentially of Na₂S and Na₂CO₃ from a soda boiler, conveyingthe smelt from a soda boiler to a dissolving tank, providing a weakliquor in a vessel that contains dissolved NaOH and CaO, the weak liquorhaving a first concentration of NaOH of not higher than 25 g/l, adding asolution of NaOH to the weak liquor in the vessel in such an amount thatthe first concentration of NaOH in the weak liquor increases to a secondconcentration of NaOH that is at least 20% higher than the firstconcentration and until a main portion of the dissolved CaO startsprecipitating into particles in the weak liquor, conveying the weakliquor and the particles to the dissolving tank, and dissolving thesmelt in the weak liquor in the dissolving tank to form a green liquor.2. The method according to claim 1 wherein the temperature of the weakliquor lies in an interval of 20-80° C.
 3. The method according to claim1, wherein the weak liquor is obtained from a process of washing asludge.
 4. The method according to claim
 1. wherein the weak liquor isobtained from a process of washing a green liquor sludge.
 5. The methodaccording to claim 1, wherein NaOH is added to the weak liquor in thevessel, such that a main portion of the CaO in the weak liquor in thevessel precipitates as particles in the vessel and that any remainingdissolved CaO precipitates in the dissolver tank on the surface of theparticles.
 6. The method according to claim 5 wherein at least afraction of the particles is removed from the weak liquor before theweak liquor is led onwards to dissolving tank.
 7. The method accordingto claim 1 wherein the NaOH that is added to the weak liquor is added atsuch a temperature that a temperature of the weak liquor is essentiallymaintained.
 8. The method according to claim 1 wherein the NaOH that isadded to the weak liquor is added at such a temperature that atemperature of the weak liquor increases.